The problem with stockings is that they can’t stuff themselves. All those candy canes and chocolate oranges at the checkout line — why not pick one up as a sign of your exceptional generosity? Here’s the snag: Those extra little gifts can actually diminish the value of the main present you bought.

It’s Christmas morning, and you open a box to find a cashmere sweater. The sweater feels plush and expensive, and you feel grateful. But then, underneath the sweater, you find a packet of Hershey’s kisses. Technically, you have been given more, but now the sweater may seem a little less exciting. Why?

An academic team based at Virginia Tech has dubbed this phenomenon The Presenter’s Paradox. The team of three psychologists — led by marketing professor Kimberlee Weaver — demonstrated the paradox recently in several studies set to be published in the Journal of Consumer Research. In the first one, a group of 54 consumers was given a choice between an iPod Touch or an iPod Touch with one free song download. On average, the consumers were willing to pay $108.41 for the iPod Touch but just $86.16 for the iPod Touch and the download.

In another experiment, the researchers asked a group of students whether they would rather receive a $1,750 tuition credit or the same credit plus $15 to use for textbooks. The 102 students who evaluated the options were significantly less likely to choose the second one.

What’s happening here? Weaver and her colleagues theorize that we average our choices. “Mildly favorable information dilutes the impact of highly favorable information,” they write. When you receive multiple gifts, you engage in what the researchers call “piecemeal processing”: you evaluate each gift separately. And so anything chintzy brings down the perceived value of even the nicest present. In short, buy something nice and let it stand for itself.